The present invention generally relates to a continuously variable transmission, and more specifically to a method and apparatus for preventing hunting between ranges in a continuously variable transmission.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
Conventional transmissions operate over a distinct number of gear ratios. Shifts between adjacent gear ratios generally keeps an engine, which drives the transmission, within its desired operating range. A drawback to a continuously variable transmission is that transmission ratios, or the ratio of transmission output speed to engine speed, can be commanded which are outside of the desired operating range of the engine which drives the transmission. If a transmission ratio is too large, then the engine may run below its desired operating point or lug limit. Therefore, it is desirable to place an upper limit on the transmission ratio command based on a lug limit of the engine to prevent possible engine stalls. Similarly, it is desirable to place a lower limit on the transmission ratio command based on an overspeed limit of the engine to prevent possible engine from running too fast.
However, during an upshift from one gear range to another, it is common that the output speed of the transmission may drop due to a slow shift or shifting away from the optimum shift point. During an upshift, if the actual output speed drops, the commanded transmission ratio can drop because the commanded ratio is limited by the output speed (which could decrease, thus decreasing the limited ratio command). Thus, the limited ratio command may cause the transmission to execute a downshift soon after an upshift due to the decrease in ratio command. During a downshift, the output speed of the transmission may increase due to shifting away from the optimum shift point. If the actual output speed increases, the commanded transmission ratio can increase because the commanded ratio is limited by the output speed (which could increase, thus increasing the limited ratio command). Thus, the limited ratio command may cause the transmission to execute a upshift soon after an downshift due to the increase in ratio command. Any series of shifting, or hunting, between one gear and another is undesirable as each shift causes a torque interruption which can decrease the perceived performance of the work machine.
What is needed therefore is a method and apparatus for preventing hunting between ranges in a continuously variable transmission which overcomes the above-mentioned drawbacks.
In accordance with a first aspect of the present invention, there is provided an transmission assembly. The transmission assembly includes a continuously variable transmission driven by an input shaft at an input speed, a mechanical transmission coupled to the continuously variable transmission for selecting a first gear range and a second gear range, an output shaft driven by the mechanical transmission at an output speed, and controller operable to receive a speed command and generate a transmission ratio command which control a ratio of the output speed to the input speed, and generate a shift command which causes the mechanical transmission to shift from the first gear range to the second gear range. An upper transmission ratio limit defined by an engine lug limit. A lower transmission ratio limit is defined by an engine overspeed limit. The controller causes the transmission to execute a shift from the first gear range to the second gear range which causes an instantaneous change in output speed. The engine lug limit or the engine overspeed limit are adjusted to inhibit a subsequent shift from the second gear range to the first gear range.
In accordance with a second aspect of the present invention, there is provided a method for controlling a transmission assembly. The transmission assembly has a continuously variable transmission driven by an input shaft at an input speed, a mechanical transmission coupled to the continuously variable transmission for selecting a first gear range and a second gear range, an output shaft driven by the mechanical transmission at an output speed, and a controller. The controller is operable to receive a speed command and generate a transmission ratio command which control a ratio of the output speed to the input speed and generate a shift command which causes the mechanical transmission to shift from the first gear range to the second gear range. The method includes the steps of defining an upper transmission ratio limit with an engine lug limit and defining a lower transmission ratio limit with an engine overspeed limit. The method further includes the steps of executing a shift from the first gear range to the second gear range which causes an instantaneous change in output speed and adjusting the engine lug limit or the engine overspeed limit to inhibit a subsequent shift from the second gear range to the first gear range.